How come cutoff voltage of 6s higher than 18v will damage the RX8 esc?
 

I have notice the message form Hotwire of RX8 when setting the cutoff voltage.
Besides, external BEC is required when running 6S.
Is that a "must" or just a prevention of potential damage?
I am running 6s 1550kv without an external bec and cutoff voltage is 20.94v.
Could that cause damage to RX8?